Our goal is to understand further the genetic and development basis of human X-linked juvenile retinoschisis (RS) (#31270 in McKusick, 1992) and the biology of Muller cells (retinal glial cells) that are believed to be involved in RS. This proposal presents the positional cloning strategy that we are using to identify and clone the RS gene. Retinoschisis causes significant loss of vision from a young age from macular cysts that are present at birth. In the peripheral retina the inner layers peep from the outer layers peel from the outer layers like sheets of tissue paper and can lead to unrepairable retinal detachment. The carrier state is not detectable, and cloning the RS gene will immediately benefit genetic counseling. The biological importance of RS extends beyond the clinical implications and involves elucidating mechanisms of retinal pathology presumed to be caused by Muller cell dysfunction. RS presents a unique opportunity to study inner retinal pathophysiology, since, unlike most other human inherited retinal dystrophies that affect the outer retinal layers, RS affects the proximal retina. In later age, RS males typically show secondary atrophy of the retinal pigment epithelium, and RS may provide insight into mechanisms important for dry forms of Age-Related Macular Degeneration. Muller cells impart structural integrity to the retina and mediate the retinal extracellular ionic homeostasis that is critical for function and survival of retinal neurons. Understanding the genetic and biological basis of RS holds promise of new understanding of basic mechanisms important for vision and retinal function. Our preliminary RS genetic study involved linkage with seven large families for which flanking RFLP markers were identified in p22.1-p22.3 (Sieving et al, 1990). We have now collected DNA from 35 RS pedigrees that include 102 affected males and more than 250 scorable meioses. We are cloning a portion of Xp22.1-22.3 into YAC contigs, preparatory to elaborating a micro-scale genetic and physical map around the RS locus. This project participants in the Michigan Human Genome Initiative Center which provides access to specialized technical support and resources. We have screened YAC libraries at the Michigan Genome Center using the RFLP marker closest to RS and have obtained four different YAC clones which we are now characterizing to construct a YAC contig around RS. We will then screen the contig for new microsatellite markers for genetic testing against RS. We are laos using a candidate gene approach by testing cDNAs in the RS region from a human retinal library enriched for X-chromosomal cDNAs.